Not Applicable
Not Applicable
1. Field of the Invention.
This invention relates to applications of optically coupled electronic integrated circuits, and more particularly to applications in which two physically and electrically isolated silicon integrated circuits may each contain either a single LED or a plurality of LEDs and also either a single corresponding light detector or a plurality of corresponding light detectors. The LEDs are integrated onto the silicon substrate and may be fabricated by any number of means such as porous silicon, avalanching silicon PN junction, forward biased silicon PN junction, deposited silicon carbide junction, light emitting polymer, or deposited GaAs. Furthermore, these applications can be realized using low efficiency, silicon based LEDs
2. Prior Art.
Traditional opto-couplers are made using an GaAs LED and a silicon detector. In the simplest opto-couplers the detector is a single device such as a diode, a bipolar transistor, an SCR, or a Triac. Detector chips may also include circuits such as amplifiers and various types of output buffer/drivers. Moreover, an additional silicon chip can be added as a input buffer/driver for the LED. The input signal may be, for example, a TTL type which can not directly drive the LED. Since the LED diver chip must be isolated from the detector chip, three separate chips are thus required in this case: the silicon LED driver, the LED, and the detector chips.
Linear opto couplers have also been made which can transmit a voltage or a current level to an output from an isolated input.
The simple LED-detector opto-couplers require a reasonably efficient LED since the light must activate the detector which is also a switch without powered amplification. For example, the detector/switch may be a floating base bipolar transistor or a floating gate SCR. Light from the LED must provide enough photo generated base current to turn the bipolar transistor xe2x80x9cOnxe2x80x9d. In another application involving a simple switch a MOSFET is turned xe2x80x9cOnxe2x80x9d by applying to the gate a photo voltage generated by a series of diodes illuminated by the LED. Unfortunately, these applications are not well suited for low efficiency, xe2x80x9conxe2x80x9d silicon chip LEDs.
Some opto-coupler applications can make use of LEDs with much less efficiency if powered amplification is available for the detector output. Thus, low efficiency on silicon chip LEDs can find useful applications if the lower speed and increased amplification can be tolerated. There can be inherent cost savings in using on silicon chip LEDs if opto-coupler applications require circuits both on the LED side as well as on the detector side since only 2 and not 3 chips are needed. The biggest cost savings are applications which require multiple optical channels between two silicon chips. In these applications putting several discrete GaAs LEDs as well as at least two silicon chips in a package is not as cost effective as putting just two silicon chips in a package with on silicon LEDs.
Some potential applications for the on silicon LED include isolated linear amplifiers, isolated line drivers such as an RS232 driver, microprocessors with isolated I/Os, and isolated switches and switch arrays. Because of the low quantum efficiency of on chip silicon LEDs, to date none of these applications have been realized by industry in spite of the physical ability to do so.
It is the objective of this invention to show how low efficiency, on chip silicon LEDs can be used to realize various types of optically isolated circuits. Specifically, these circuits include a linear isolation device for analog signals, an isolated A to D converter, an isolated D to A converter, isolated buffer/driver circuits, isolated multiplexers, and isolated switches. Also, a microprocessor is proposed with optically isolated I/O ports, A to D and D to A converters, a multiplexer, and solid state switches. These functions can be achieved using two silicon chips which optically communicate data back and forth. Isolation is achieved by placing a transparent, insulating barrier between the two chips through which light is transmitted.